Spatial light modulators (SLMs) typically consist of an array of electronically addressable pixel elements and related control circuitry. A frequently used type of SLM is the digital micromirror device (DMD), in which each pixel element is a any micro-mechanical mirror, capable of independent movement in response to an electrical input. Incident light is modulated by reflection from each pixel. A typical application is for image display, where light from each pixel is magnified and projected to a display screen by an optical system.
DMDs can be fabricated in many different forms including the cantilever beam, hinge, and torsion beam embodiment. While the disclosed invention is equally applicable to all forms of DMDs, specific examples will reference the torsion beam digital micromirror as disclosed in U.S. Pat. No. 5,061,049, entitled "Spatial Light Modulator and Method" assigned to the same assignee as the present application.
For many applications, the SLM is binary in the sense that each pixel element may have either of two states. The element may be off, which means that it delivers no light. Or, the element may be on, which means that it delivers light at a maximum intensity. To achieve a viewer perception of intermediate levels of light, various pulse width modulation techniques may be used. Some of these techniques are described in pending U.S. Pat. No. 5,278,652, issued Jan. 11, 1994 entitled "DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System" assigned to the same assignee as the present application.
In general, pulse width modulation produces an integrated brightness by switching each pixel on or off during each frame for a period that corresponds to a binary number. Pulse width modulation typically uses as "bit-frame" loading, in which data for every pixel in a frame is loaded into a memory cell associated with each pixel. One bit of data is loaded into each memory cell in the array and then all pixel elements are set to correspond to that bit-frame of data. During the display time of the current bit-frame, data for the next bit-frame is loaded. According to one pulse width modulation method, the most significant bit is displayed for 1/2 of a frame period, the second most significant bit for 1/4 frame period, etc., with the least significant bit (LSB) representing a display time of 1/(2.sup.n &lt;1) frame period, for n-bit brightness quantization. Therefore, for 8-bits of pixel brightness quantization, the SLM is loaded eight times per frame, one bit-frame at a time.
While this is an efficient method of creating a wide range of brightness levels, it has the disadvantage of requiring a very high data transfer rate during the LSB display period. For an 8-bit data word there are 8 bit-flames of data that must be loaded during one frame period. Pulse width modulation requires that 1/8 of the data for an entire frame period must be loaded during 1/255 (1/(2.sup.8 -1)) of the frame period. This peak data rate is limited by the number of pins available to transfer data and the data frequency on those pins. A high peak data rate translates into a high pin count and/or high frequency, which increases device and/or system costs. A need exists for a way to reduce this peak data rate.